Drug delivery devices allowing for multiple dosing of a required dosage of a liquid medicinal product, such as liquid drugs, and further providing administration of the liquid to a patient, are as such well-known in the art. Generally, such devices have substantially the same purpose as that of an ordinary syringe.
Pen-type injectors of this kind have to meet a number of user specific requirements. For instance in case of those with diabetes, many users will be physically infirm and may also have impaired vision. Therefore, these devices need to be robust in construction, yet easy to use, both in terms of the manipulation of the parts and understanding by a user of its operation. Also, the dose setting must be easy and unambiguous and where the device is to be disposable rather than reusable, the device should be inexpensive to manufacture and easy to dispose. In order to meet these requirements, the number of parts and steps required to assemble the device and an overall number of material types the device is made from have to be kept to a minimum.
The medicinal product to be dispensed by means of the drug delivery device is typically provided in a disposable or replaceable cartridge, such as a vial, ampoule or carpule comprising a slidably disposed piston to be operably engaged with a piston rod of a drive mechanism of the drug delivery device. By applying thrust to the cartridge's piston in distal direction, a predefined dose of the liquid drug can be dispensed and expelled from the cartridge.
Cartridges as they are typically used with drug delivery devices, in particular with pen-type injectors are typically sealed by means of as septum. Such a septum is commonly designed as a rubber stopper providing an air-tight seal but being pierceable by piercing elements such as needles or cannulae.
Typically, such drug delivery devices comprise a cartridge holder adapted to receive a cartridge which is hermetically sealed with such flexible and deformable septum. At its lower and distal end section, the cartridge holder can for instance be threadedly engaged with a needle mount. Said needle mount or needle holder typically comprises a correspondingly threaded cylindrical portion for releasably interconnecting needle holder and cartridge holder.
During assembly of the needle holder the proximally located tipped end of the needle penetrates the septum of the cartridge. In this way, a fluid-transferring connection for the purpose of dose dispensing can be established.
In particular, during dispensing of a dose of the medicinal fluid contained in the cartridge, a respective fluid pressure is built-up. During a dose dispensing procedure the piston is subject to an axial squeezing and/or the septum might become subject to an axial expansion. Due to their elastic properties, septum and/or piston may store elastic energy during dose dispensing. Right after completion of a dose dispensing procedure, the septum and/or the piston typically relax to their initial configuration, because the fluid pressure inside the cartridge drops. Since the piston is in an abutment position with a distally arranged bearing of a piston rod, relaxation of the piston inevitably leads to an expansion of the piston in distal direction. Similarly, the expanded section of the septum retracts into the cartridge.
Both relaxing phenomena may in turn lead to a post-dispensing built-up of a non-negligible fluid pressure and, as a consequence, a certain amount of medicinal fluid may be supplementary expelled from the cartridge, which can be typically observed in the form of droplet formation at the distal tip of the needle, which remains in permanent fluid-transferring contact with the inner volume of the cartridge as long as the needle assembly remains connected with the cartridge holder.
Additionally, such droplet formation may occur during assembly of the needle mount to the cartridge holder, especially, when the inside pressure of the cartridge is larger than ambient pressure. Since some dispensable medicinal products like Insulin or Erythropoietin (EPO) have to be stored refrigerated, a non-negligible pressure increase may for instance arise due to thermal expansion, e.g. if the cartridge or the drug delivery device is kept in a non-refrigerated environment at least for a while.